Air ducts are known for use in automotive applications for a number of purposes. For example, they are used to transfer filtered air from an automotive air filter through the air intake system of an internal combustion engine. They are also used to transfer temperature modified air from within the engine compartment to the passenger compartment.
Air ducts are often formed using two different approaches. One approach involves low cost conventional blow molding. A second approach involves a high cost intermittent dual durometer extrusion. As a compromise, it is known to mold a polymeric material to a blow molded duct. In one teaching a blow mold is provided with at least one embedded knife. A tubular body is formed by means of blow molding which includes an apex adjacent where the embedded knife is located. The knife is used to remove the bubbled end formed during the blow molding process. Once the bubbled end is removed, the remaining tubular body includes an inwardly directed lip at an open end. The open end with inwardly directed lip is inserted onto a mandrel and a polymeric material is injection molded around the tubular body and the mandrel. The inwardly directed lip is taught to provide a compressive sealing force on the mandrel to provide a unique sealing arrangement.
While the latter approach provides a compromise between conventional blow molding and dual durometer extrusion, the use of an inwardly extending lip is highly undesirable as consistent wall thickness is difficult to maintain due to process variations. In particular, there is a potential for undesirable separation of the tubular body and the polymeric material in part because of the limited area of contact between the lip and the polymeric material.
Further, attempting to injection mold over a blow molded component is not easy. Blow molded components tend to have an interruption created at a seam between opposing blow molds, providing a ready pathway for leakage. When polymeric material is injection molded to a blow molded tubular body, high pressures are obtained. If a uniform seal cannot be obtained between the injection mold and both the tubular body and the mandrel, such an interruption and the leak pathway it provides will reduce the pressure that must be maintained within an injection mold cavity and provide a way for the injection molded material to escape. The use of an inwardly directed lip compressively engaging a mandrel prior to an injection molding step provides, only limited sealing potential. At best, a seal is only assured at the point of contact between the lip end and the mandrel and only in the case of relatively thin walled air ducts (below 2.5 mm). More likely, leakage is still likely since the interruption formed during blow molding is not eliminated.